We have examined the hypothesis that submembrane granular fusion to cell membranes is driven by granular osmotic pressure, vesicular swelling, and membrane performation at the point of contact between granular vesicle and cell membrane. WE examine sea urchine egg cortical granule exocytosis with differential interference contrast light microscopy, phase contrast microscopy, electron microscopy of rotary-shadowed rapidly-frozen freeze-fracture replicas, and intracellular recording to measure cell capacitance and potential. We find that exocytosis is prevented when the osmolality of the medium surrounding the eggs is raised from 1 to 2 osmol kg-1. High osmolality also prevents calcium-dependent exocytosis in vitro. Prior treatment with calcium at high osmolality triggers fusion when normal osmolality is restored, even if calcium is removed before dilution. Addition of calcium causes the cortical granules to swell, as revealed by computer image analysis. The large increase in membrane capacitance which normally accompanies fusion is absent in eggs activated in solutions of high osmolality. Our data are consistent with the idea that a secretory granule must swell to fuse with the plasma membrane and support the hypothesis of an osmotically driven fusion step during exocytosis.